Chancellor Alistair Darling listens to a speech by Mervyn King. He also reads rather a lot of letters from the Bank of England governor. Photograph: Lewis Whyld/PA

Back during what economists called the "Great Stability," when Gordon Brown and his chum Ed Balls drew up the rules for Bank of England independence, they envisaged it being rare for its governor to have to write to the chancellor because inflation was more than 1% off target. That was true for the first decade but today's missive – with inflation at 3.5% – was the sixth fired off to No 11 since 2007.

What King and his fellow central bankers are discovering – as the International Monetary Fund pointed out last week – is that strictly following the rulebook and keeping inflation at around 2% during the good times hasn't left them enough room for manoeuvre when things turn scary. Once interest rates have hit rock bottom, central bankers have run out of road. They can resort to unconventional measures such as quantitative easing but they are risky and largely untested. As IMF economist Olivier Blanchard mused last week, would it have been so bad to let inflation jump to, say, 4%, if that meant policymakers could have made a better job of responding to the worst financial crisis since the war? The risk is that the same discredited rule-book makes the hawks on the monetary policy committee itch to rein in quantitative easing and push up rates too soon, while businesses are still coping with a credit drought, and consumers wrestling with the legacy of a decade-long borrowing binge.

To cast Blanchard's question forward, would it be so bad to grit our teeth and tolerate 4% inflation if it meant protecting the economy from sliding into the abyss?